The present invention relates to a process for the selective dimerization of i-butene present in a C4 raffinate I and comprises the steps of a first hydroisomerization of the raffinate I, a subsequent removal of 2-butene and n-butane by distillation in a column and the selective dimerization of part of the i-butene-containing product from the top of the column.
The dimers of i-butene are 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene. They still each contain a double bond and can be passed to the oxo process or to esterification, for example to produce isononyl alcohol from diisobutylene (DIB) and then dinonyl phthalate or, in the case of direct esterification of the DIB, dioctyl phthalate which are used as plasticizers for thermoplastics. Triisobutene can also be processed to produce dodecyl mercaptan, a rubber auxiliary. Finally, mention may be made of the addition of DIB or octanes derived therefrom by hydrogenation to motor fuel.
It is already known that i-butene can be oligomerized over many electrophilic catalysts (Olah, Molaur, Hydrocarbon Chemistry, Wiley, 1995). For example, processes using acidic ion exchangers or pentasil zeolites have been described. A disadvantage of these methods is the low selectivity to DIB, since the processes always form not only the main product but also higher oligomers of i-butene, namely triisobutene (C12), tetraisobutene (C16), pentaisobutene (C20) and so forth, whose proportions decrease further with ever increasing degree of oligomerization. Such a process has to be followed by a separation of the components formed.
If the feed stream contains not only i-butene but also 1-butene or 2-butene, the selectivity to DIB decreases further, since C8 products are simultaneously formed from i-butene and 1-butene or 2-butene (known as codimers) at high conversions.
Owing to the close proximity of their boiling points to that of DIB, these cannot be separated off at justifiable cost and thus represent a loss source in the process.
For this reason, processes which have a very high selectivity to DIB are sought. One way of increasing the selectivity is high dilution of the i-butene by octanes and oxygen compounds as disclosed in U.S. Pat. No. 5,877,372. However, unreacted i-butene and inert substances have to be separated off and circulated in this process.
Another route is described by EP 0 008 860 A1, where the catalyst is installed in wire mesh pockets in a distillation column. In this process, raffinate I having an i-butene content of about 50% and a 1-butene content of 25% is processed at low pressures. Despite this, the codimers which are virtually impossible to separate off are the major by-products. In addition, low i-butene conversions and thus i-butene losses have to be accepted in this process.
EP 0 850 904 A1 describes the separation of 1-butene and 2-butene from i-butene in C4 raffinate I by appropriately linking hydroisomerization reactors with various distillation columns. However, the object here is to isolate a very pure i-butene stream.
DE 196 46 405 A1 describes the selective oligomerization of i-butene from a 1-/2-butene-containing C4 stream, with the particularly troublesome 1-butene being converted into 2-butene by hydroisomerization and the 2-butene being separated from the i-butene by distillation. The resulting i-butane/i-butene stream is oligomerized over a heterogeneous acid catalyst in a reactor. Virtually no codimers which are difficult to remove are formed, but C12-oligomers constitute the main product.
In view of this background, it is an object of the present invention to improve the known processes so that DIB can be prepared in high conversions and with high selectivity from a stream comprising i-butene, 1-/2-butene and butanes.
This object is achieved by a process having the features specified in claim 1.
The process for preparing diisobutylene (DIB) from an i-butene-containing C4 raffinate I comprises the steps
a) subjecting the C4 raffinate I to a first hydroisomerization over a noble metal catalyst, with this being carried out at 30-90xc2x0 C., preferably 50-60xc2x0 C., at 5-30 bar, preferably 10-20 bar, at an LHSV of 1-30 hxe2x88x921, preferably 5-30 hxe2x88x921, and using 3-20 standard liters of gaseous hydrogen per liter of liquid C4raffinate I, which is above the amount of hydrogen required for the hydrogenation of highly unsaturated compounds in the C4 raffinate I;
b) fractionally distilling the hydroisomerization product from a) in a fractionation column, with this being carried out at 5-25 bar, preferably 8-12 bar, and the temperature established under this pressure and 2-butene and n-butane being taken off as bottoms and i-butene and i-butane being taken off at the top;
c) optionally taking off a side stream from this fractionation column above the inlet, preferably at the level of the middle of the column, and subjecting it to a second hydroisomerization over a noble metal catalyst, with this hydroisomerization being carried out at 30-90xc2x0 C., preferably 50-60xc2x0 C., at 5-30 bar, preferably 10-20 bar, at an LHSV of 1-30 hxe2x88x921 and using 3-20 standard liters of gaseous hydrogen per liter of C4 raffinate I and the hydroisomerization conditions in a) and c) being identical or different; with the hydroisomerization product being recirculated to the fractionation column used under b), preferably above the offtake point;
d) condensing the product from the top of the column in b), dividing it into two parts and subjecting the first part to a third hydroisomerization over a noble metal catalyst, with this hydroisomerization being carried out at 30-90xc2x0 C., preferably 50-60xc2x0 C., at 5-30 bar, preferably 10-20 bar, at an LHSV of 1-30 hxe2x88x921, preferably 5-30 hxe2x88x921, and using 0.3-20 standard liters of gaseous hydrogen per liter of condensate and the hydroisomerization conditions in a) and d) being identical or different and the hydroisomerization product from d) being recirculated to the fractionation column of b), preferably into its upper third;
e) introducing the second part of the condensed product from the top of the column in b) into a reactive column in which the C4-hydrocarbons are vaporized preferably at 50-100xc2x0 C. and are passed over an acidic heterogeneous catalyst over which the i-butene is dimerized, with the dimerization being carried out at 40-100xc2x0 C., at 3-30 bar, preferably 5-20 bar, and an LHSV of 5-50 hxe2x88x921. The catalyst can be present in a special column packing, e.g. in a commercially available packing of the type MONTZ Multipak I which allows separation of the DIB from the feed stream. Thus, inert C4-hydrocarbons can be condensed at the top of the reactive column, with part of this condensate being able to be recirculated to the column to remove the heat of reaction and the other part being able to be taken off, and dimerization products, i.e. C8-hydrocarbons, being able to be taken off at the bottom of the reactive column.
In the process of the invention, the C4 stream is thus largely freed of 1-butene and 2-butene and subsequently dimerized over an acidic catalyst installed in a reactive column. The feed stream to this reactive column contains butanes in addition to i-butene, which make the reaction temperature in the fixed bed controllable by removal of the heat of reaction. The process of the invention differs from the process known from DE 196 46 405 A1 in, in particular, the steps c) (optional) and e).
As i-butene-containing C4 mixture, it is possible to use a C4 raffinate I which is obtained from the crude C4 distillation fraction of a cracker product by removing the 1,3-butadiene present therein by extraction and making economic use of it. This extraction also removes other highly unsaturated hydrocarbons, e.g. vinylacetylene, 1,2-butadiene and other acetylene compounds, from the crude C4 distillation fraction. The C4 raffinate I obtained after the extraction still contains minor amounts of highly unsaturated compounds. The predominant components of C4 raffinate I are n-butane, i-butane, 1-butene, 2-butene (cis and trans) and i-butene. C3- and C5-hydrocarbons may also be present in minor amounts.
The first hydroisomerization as reaction step a) of the process of the invention is carried out at a temperature of 30-90xc2x0 C., preferably 30-80xc2x0 C., particularly preferably 40-60xc2x0 C., and a pressure of 5-30 bar, preferably 10-20 bar. An LHSV (Liquid Hourly Space Velocity) of 1-30 hxe2x88x921, preferably 5-30 hxe2x88x921, particularly preferably 5-12 hxe2x88x921, is employed. The amounts of gaseous hydrogen added has to remove the abovementioned highly unsaturated compounds and also be present for the hydroisomerization. Partial hydrogenation of the highly unsaturated hydrocarbons forms monounsaturated hydrocarbons which are already present in the C4 raffinate I. The amount of hydrogen necessary to eliminate the highly unsaturated hydrocarbons depends on the proportions in which they are present in the feed and can easily be determined by analytical methods known to those skilled in the art. The additional amount of hydrogen required above this amount is 2-15 standard liters, preferably 3-10 standard liters, of gaseous hydrogen per liter of liquid raffinate I.
For the hydroisomerization, C4 raffinate I is in the liquid state. It can be passed over the catalyst from the top downwards in the trickle mode, or the catalyst-containing reactor is supplied from the bottom upwards in the flooded mode. The gaseous hydrogen can be conveyed in cocurrent or in countercurrent to the C4 raffinate I. The reactor is preferably operated in the flooded mode and the hydrogen is preferably passed through it in cocurrent.
All noble metal hydrogenation catalysts are suitable for the hydroisomerization. Possible noble metals are Ru, Rh, Pd, Ir, Pt, preferably Ru, Pd, Pt, particularly preferably Pd. As support materials to which the noble metals may be applied, it is possible to use Al2O3 in various modifications, SiO2, carbon, kieselguhr, BaSO4 and other salts. It has been found to be useful to regulate the activity of the noble metals of the abovementioned type by addition of sulphur compounds.
In reaction step b) of the process of the invention, the hydroisomerization product from a), which now comprises i-butene and 2-butene as main components, is fractionally distilled. This is carried out at a pressure of 5-25 bar and the temperatures established under the distillation conditions. Typically, at a particularly preferred column pressure of 11 bar, the temperature established at the top of the column is 73xc2x0 C. and that established at the bottom of the column is 78xc2x0 C. It is also preferred that the reaction steps a), b), c) and d) of the process of the invention are carried out at the same pressure, and the temperatures are in each case matched to requirements.
Above the point at which the hydroisomerization product is fed into the distillation column a substream of the product is preferably taken from the column and, in step c), subjected to a second hydroisomerization in a side stream reactor. As in the first hydroisomerization step, preference is given to using an LHSV of 1-30 hxe2x88x921, particularly preferably 5-25 hxe2x88x921. The liquid can be passed through the reactor from the top downward or preferably from the bottom upward in the second hydroisomerization step too, with the hydrogen likewise being able to be conveyed in cocurrent or in countercurrent. The amount of hydrogen introduced can be lower than in the first hydroisomerization, since diene compounds no longer have to be expected in the second reactor. The product is returned to the column immediately above the offtake point. The 2-butene formed in the second hydroisomerization step goes to the bottom of the column. Offtake and reintroduction of product from/to this second hydroisomerization are preferably carried out at the half height of the distillation column described under b).
The bottom product obtained from reaction step b) is a mixture of mainly n-butane and 2-butene. The amount of 2-butene is made up of the 2-butene originally present in the C4 raffinate I and the 2-butene formed by hydroisomerization of 1-butene.
The product taken off at the top of the distillation b) of the process of the invention consists essentially of i-butene and i-butane and minor amounts of 1-butene. It is condensed and then divided into two parts. The first part is, in reaction step d), fed to a third hydroisomerization which is likewise carried out under the conditions specified for the first and second hydroisomerizations.
The ratio of the two substreams of the condensed product from the top of the fractionation column is set so that 10-50 parts by volume, preferably 25-35 parts by volume, are fed as first part to the third hydroisomerization d), based on 1 part by volume fed as the second part to the dimerization described under e). Since this second part together with the bottoms from the fractionation column correspond to the total volume of C4 raffinate I fed to the first hydroisomerization, the flow of material through the process of the invention has superposed on it a circulation through the second and third hydroisomerization reactors and the fractionation column which corresponds in the above-described way to 10-50 times the substream fed to the dimerization.
In reaction step e) of the process of the invention, the second part of the condensed product from the top of the column is fed to the dimerization in a reactive column. As catalysts for the dimerization, use is made of heterogeneous, inorganic or organic catalysts, for example acidic zeolites, silica gels and acidic Al2O3, acidic sheet silicates and framework silicates, acid-doped support materials or gel-like or macroporous cation exchangers in the H+ form. The catalyst is present in a packing, and the C4 fraction is passed in gaseous form through the catalyst packing at temperatures of from 40 to 100xc2x0 C. The i-butene dimerizes over the catalyst, the inert butanes condense at the top of the column and are returned as runback. This runback comprising inert C4-hydrocarbons keeps the temperature over the catalyst constant by the heat of reaction being removed by vaporization of the inert butanes.